Gas pipelines and oil pipelines have been built as the basis of energy supply. Recently, many gas fields have been developed in places remote from consuming regions with increasing demand, particularly for natural gas, as the backdrop. Accordingly, pipelines have shown a tendency to be longer, and have developed an obvious tendency to have larger diameters and to be highly pressurized for mass transport.
FIG. 13 shows a flow chart of the process of pipeline construction focusing on the design of such pipelines. The conventional process of pipeline design is broadly classified into steps of (1) design of a pipeline system and (2) structural design of a pipeline. In designing the pipeline system, the type, diameter, thickness, and operating pressure of a pipe are temporally assumed such that an operating cost and a construction cost of a pipeline are minimized with consideration of transport volume and conveying distance that represent the scale of the project as prerequisites. In structural design of the pipeline, structural analysis is carried out with consideration of ground displacements and the like generated during earthquakes on the basis of a pipeline route, which is a pipeline shape to be constructed that is presumed from the strength and the dimensions of the pipe temporally set in the designing of the pipeline system and geographic features and the like of the places of construction; and then the maximum stress, the maximum strain, and local buckling are checked.
When the characteristics of the pipe temporally set in the designing of the pipeline system do not satisfy these check conditions, the process returns to the step of designing the pipeline system and the characteristics of the pipe are reset. When the characteristics of the pipe satisfy the above-described check conditions, the characteristics temporally set in the designing of the pipeline system are set as the specification of the pipe. The pipeline company then places an order with a steel company for the pipe, and the steel company manufactures the line pipe according to the specification given by the pipeline company.
In the local buckling check, it is checked whether the pipe with the conditions that have been temporally set in the designing of the pipeline system has sufficient local buckling performance to endure the maximum compressive strain and the maximum bending strain presumed under the conditions where the pipeline is constructed. Specifically, the critical local buckling strain of the pipe is obtained, and it is determined whether the critical local buckling strain is larger than the maximum strain generated in the pipeline or not.
Equation
The critical local buckling strain of the designed pipe is obtained as follows. In general, the critical local buckling strain of a pipe is represented by (critical local buckling strain)=coefficient·{(pipe thickness)/(pipe diameter)}exponent. The coefficient and the exponent in the relation are obtained by plotting experimental data of local buckling with pipes as shown in FIG. 14, by drawing curves such that the lower bound of the experimental data are enveloped, and by fitting to these lower-bound envelope curves.
Table 1 shows proposed design equations of the critical local buckling strain acquired on the basis of the above-described local buckling experiments with real pipes.
TABLE 1ReferenceExpressionS-S curveShermanεcr = 16(t/D)2(1976)Murphy andεcr = 0.5(t/D)Continuous-Langnerhardening model(1985)εcr = 0.33(t/D)Yield-plateau modelGresnigtεcr = 0.5(t/D) −(1986)0.0025 + 3000(pD/2Et)2Stephensεcr = 2.42(t/D)1.59Et al.(1991)
The proposed design equations of the critical local buckling strain shown in Table 1 prescribed by the current design standard are based on experimental data of pipes of grade X65 (grade of strength according to the API (American Petroleum Institute) standard in the United States) or lower. Therefore, the applicable scope in FIG. 13 is limited to line pipes of grade X65 or lower.
Besides those shown in Table 1, the following design equation of the critical local buckling strain is presented in “Guidelines for Anti-Seismic Design of High-Pressure Gas Pipelines (revised edition)” (issued by the Japan Gas Association, March 2000, page 39).ε=35(t/D) (%)
Since the design equations of the critical local buckling strain are acquired on the basis of the buckling experiments of pipes as described above, the critical local buckling strain is obtained on the basis of these estimate equations and it is determined whether the critical local buckling strain is larger than the maximum strain or not in the local buckling check. When the critical local buckling strain is smaller than the maximum strain, the process returns to the step of designing the pipeline system and the conditions are reset. In the resetting process at this time, the critical local buckling strain of the pipe is increased by increasing the pipe thicknesses on the basis of the relationship of (critical local buckling strain)=coefficient·{(pipe thickness)/(pipe diameter)}exponent.
The above is a case for the line pipes of grade X65 or lower for which design equations of the critical local buckling strain have been acquired. When pipes of grade X70 or higher for which estimate equations of the critical local buckling strain have not yet been acquired are adopted for a pipeline, a sample pipe is produced by way of trials and errors, and the local buckling experiment is carried out such that the critical local buckling strain of the pipe is acquired as shown in FIG. 15. Then, it is determined whether the acquired critical local buckling strain of the pipe is larger than the maximum strain. When the critical local buckling strain is also smaller in this case, a sample pipe with a larger thickness is produced again and checked as in the case of a pipe of grade X65 or lower.